pfcvo

got it about 10 minutes after a 30 second "170 mph" run somewhere in Mexico. The car started to misfire from mid to high rpms and then the check engine light flashes and then stayed on. Tried to change most of the spark plugs (couldn't reach in the #5 #8 on the driver side) and changed all of the plug wires. Drove around with same results but with popping sounds in the exhaust, and the check engine light flashes frequently. Just want to know what is going on and what to do next. Thanks.

BlackZ06

First thing I'd do - inspect the plugs that you took out. Are any appearing "oily" or "black". If so, those are the cylinders that are suspect.

Is it ONLY missing at mid- to high-RPM ??? Idle is smooth ??? If the idle is smooth your valvetrain is probably OK. Then it is either a fuel or electric problem. Are there ANY other codes besides the P0300 posted ??

If you have a rough idle, since you've already changed most of the plugs/wires, take the valve covers off and inspect the valve train. It is possible you have a dropped valve and/or other problems with the valve train.

Best of luck.

runamuk

Are you using a scanner or the cars diagnostics to read the codes?

The built in diagnostics on the C5 will never read a misfire showing anything other than a P0300, if you want to try to narrow it down further to say a P030X, X being the number of the misifiring cylinder then you'll need a scanner.

Of course it may still show a P0300 using a handheld scanner if it is indeed a multiple cylinder or the PCM is unable to determine which one is misfiring.

From the sound of what you wrote, that it happened right after a high speed run you may want to check your valve springs, that will cause the symptoms that your having.

pfcvo

Yes, at idle in neutral, it seems normal and doesn't miss, it's only when I'm in gear and I'm driving the car. I revved the engine to redline in neutral and seems normal. I highly doubt it's my valve springs. Thanks. I guess I need to retempt to replace the last 2 plugs and reseat the plug wires.